Geologic history of salt beds and related strata in the upper part of the Madison Group (Mississippian), Williston Basin, Montana and North Dakota
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Publications and source records attributed to William Jasper Sando.
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The Mississippian history of the northern Cordilleran region of the United States consists of two principal depositional cycles separated by a cycle of epeirogenic uplift and erosion. Each depositional cycle is divisible into phases that represent significant changes in depositional patterns. During Cycle I (early Kinderhookian-early Meramecian), predominantly carbonate and evaporite deposition took place on a broad cratonic shelf bordered on the east by land and on the west by a deep trough that received terrigenous sediments from an adjacent western landmass. Kinderhookian transgression was followed by regression during the Osagean and early Meramecian. Regional uplift during latest early Meramecian time (Cycle II) drained the shelf area and caused the sea to be confined to the western trough. During Cycle III (middle Meramecian-Chesterian), the sea again transgressed onto the craton, which was differentiated into the Big Snowy-Williston, Wyoming, and Uinta basins, where terrigenous and carbonate sediments were deposited. The Big Snowy-Williston basin was uplifted during latest Chesterian time and lost its identity, but the Wyoming basin continued to expand into the Pennsylvanian, when it engulfed most of the Cordilleran platform and breached the Transcontinental arch. Application of generalized carbonate, terrigenous, and evaporite depositional models results in specific models that explain the Mississippian depositional patterns.
Morphologic and stratigraphic evidence bearing on the phylogenetic relationships of aulate Carboniferous corals suggests that species previously assigned to the genus Aulina Smith represent at least six different lineages that descended from three different ancestral genera. A classification designed to better reflect phylogenetic relationships includes the new genera Aulokoninckophyllum , Aulostylus , Solenodendron , and Vesiculotubus . The new North American taxa Aulostylus tubiferus tubiferus , A . tubiferus eotubiferus , and A . bamberi are described and illustrated. Two new Chinese species, Aulina ( Pseudoaulina ) loi and A . ( P .) sinensis , are also proposed.
New paleontologic evidence requires a revision of previous interpretations of the stratigraphy of Mississippian sequences in the Cordilleran miogeosyncline of eastern Idaho and northeastern Utah. A postulated unconformity between rocks of early Osagean age and rocks of middle Meramecian age is no longer tenable in the light of new data. The new evidence supports continuous deposition from the Lodgepole Limestone (Kinderhookian and early Osagean age) into the Little Flat Formation (early Osagean to middle Meramecian age) and its equivalents, and the interpretation of a phosphatic siltstone and shale interval in the lower part of the Little Flat Formation as a starved-basin facies.
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The youngest strata recognized in the Madison Limestone arc dated by foraminifers and corals at two localities on the west flank of the Bighorn Mountains in Wyoming and Montana. Fossils collected in situ from the Madison at Shell Canyon represent Zone 12 of Mamet and Skipp of late Salem age (middle Visean), previously thought to be unrepresented by strata in the northern Cordilleran region of the United States. A block of residual chert found at Devils Canyon contains fossils with a possible age range of microfaunal Zones 12 15 (late Salem to Ste. Genevieve, middle to late Visean), providing further evidence of an extended age range for Madison strata prior to post-Madison, pre-Amsden erosion.
The Lower Ordovician series (Beekmantown group) is at least 3500 feet thick near Chambersburg, Pennsylvania, approximately 1200 feet thicker than was earlier estimated. Reverse faulting in the upper part of the section may have obscured the youngest Early Ordovician and oldest Middle Ordovician beds in the area, so that the total thickness of the Lower Ordovician series may be comparable to the thickness measured in Maryland. The Beekmantown group is represented by the Stonehenge limestone and the Rockdale Run formation. Moving the base of the Stonehenge downward to include about 200 feet of strata previously referred to the underlying Conococheague formation facilitates regional correlation without impairing mappability. Newly discovered fossils from the Rockdale Run formation add notably to the faunas of the upper part of that formation. Comparison of a trilobite-brachiopod faunule near the top of the Diparelasma zone (Cotter-Powell equivalent) with similar fossils in Zone J of the Utah-Nevada sequence suggests that Zone J may be no younger than the Powell dolomite of the Ozark region. Difficulty in recognizing correlatives of the Smithville and Black Rock formations suggests the need for restudy of these units in the Ozark region. Evidence from the Chambersburg section neither proves nor contradicts the hypothesis of continuous deposition from Early Ordovician to Middle Ordovician time in the central Appalachian area.
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